This application relates generally to systems and methods for compensating for image defects in imaging systems, particularly photoreceptor ghosting image defects in xerographic imaging systems.
Photoreceptor ghosting is a problem that plagues many xerographic printing systems. Generally, ghosting is caused by charges trapped in a photoreceptor during an imaging cycle that occurs prior to a present imaging cycle. Typically, the charges trapped in the photoreceptor are holes. Also typically, this problem occurs during exposure or transfer. Erase can also play an important role. Typically, the trapped charges (holes) are released during a subsequent imaging cycle. This release of charges (holes) trapped in the photoreceptor during a prior imaging cycle creates a ghost of the previous image on a subsequent image.
Thus, a ghost defect has a functional relationship to the image captured by the photoreceptor during a previous imaging cycle. A ghost defect is also dependent on a state of the photoreceptor. An example of a state of the photoreceptor that affects a ghost defect is the age of the photoreceptor. A new photoreceptor and an old photoreceptor will not typically evidence an identical ghost defect given the same prior image.
Several variables affecting the configurations of a xerographic printing device also affect the appearance of a ghost defect. For example, the charging level of the device, the exposure level of the device, the transfer set points of the device, and so on, all have an impact on the appearance of a ghost defect in an image created by the device.
A ghost defect typically occurs at a spatial distance from the original image giving rise to the ghost defect equal to the circumference of the photoreceptor. This spatial distance corresponds to the rotation of the photoreceptor. When the photoreceptor rotates exactly one rotation, any residual charge of the previous image on the photoreceptor results in a ghost defect on the current image created by the photoreceptor.
Although the degradation of a ghost defect in the photoreceptor charge is fairly rapid, such defects can exist in an image produced some multiple of revolutions of the photoreceptor other than one. In other words, a ghost defect could appear at a spatial distance equivalent to twice the circumference of the photoreceptor from the image giving rise to the ghost. The typical spatial distance of one revolution of the photoreceptor or one times the circumference of the photoreceptor is also referred to at times as the ghost distance.
Ghost defects are unwanted imperfections in an image created by the device. Thus, ghost defects can be extremely objectionable to the user of a xerographic system. It is believed that ghosting defects are a critical problem for both belt photoreceptors and drum photoreceptors. There is not any known method or system for eliminating or controlling ghost defects that is able to eliminate or control ghost defects in a robust manner.
There are two forms of a ghost defect. A negative ghost defect exists where the ghost image is lighter than the surrounding image. A positive ghost defect exists where the ghost image is darker than the surrounding image.
It is believed that a root cause of ghost defects is associated with defects in the structure of a photoreceptor. Nevertheless, the appearance of a ghost defect is often triggered by an interaction between the photoreceptor and the xerographic imaging process.
In some instances, regions where charges are trapped on the photoreceptor and then released in creating a ghost image are charged higher (more positive) with respect to the normal surrounding regions. In exposure induced ghosting, the trapped charges are created in an image-wise fashion. In transfer induced ghost defects, the trapped charges are created in an anti-image-wise fashion. Thus, the result of the release of the trapped charges is either a positive or a negative ghost of the previous image. The ghost image is typically observed in halftone areas where the difference in the charge between the trapped charges and the surrounding normal region is evidenced as either a growth or attrition of the halftone dots.
When there is an attrition of the halftone dots, the halftone dots are smaller than the halftone dots in the surrounding image region. This corresponds to a negative ghost image. When the difference in the charge results in a growth of the halftone dots, the halftone dots in the area of the ghost defect are larger than the halftone dots in the surrounding normal image region. This corresponds to a positive ghost image.
It is believed to be likely that certain photoreceptors are predisposed to exhibiting ghost defects. However, despite this predisposition in certain photoreceptors, the presence or absence of ghost defects in images created by a given photoreceptor may evidence themselves and then disappear periodically over the life of the photoreceptor.